Add Smrnmi extension missing instructions

Signed-off-by: Afonso Oliveira <[email protected]>

arch/inst/Smrnmi/mnret.yaml

@@ -0,0 +1,21 @@
+# yaml-language-server: $schema=../../../schemas/inst_schema.json
+
+$schema: inst_schema.json#
+kind: instruction
+name: mnret
+long_name: No synopsis available.
+description: |
+      No description available.
+definedBy: Smrnmi
+assembly: mnret
+encoding:
+  match: '01110000001000000000000001110011'
+  variables: []
+access:
+  s: always
+  u: always
+  vs: always
+  vu: always
+data_independent_timing: false
+operation(): |
+